Molybdenum
''' Molybdenum '''is a chemical element with the symbol Mo and atomic number 42. The free element, which is a silvery metal, has the sixth-highest melting point of any element. It readily forms hard, stable carbides, and for this reason it is often used in high-strength steel alloys. Molybdenum does not occur as the free metal in nature, but rather in various oxidation states in minerals. Industrially, molybdenum compounds are used in high pressure and high temperature applications, as pigments and catalysts. Most molybdenum compounds have low solubility in water, but the molybdate ion MoO42− is soluble and will form if molybdenum-containing minerals are in contact with oxygen and water. Recent theories suggest that the release of oxygen by early life was important in removing molybdenum from minerals into a soluble form in the early oceans, where it was used as a catalyst by single-celled organisms. This sequence may have been important in the history of life, because molybdenum-containing enzymes then became the most important catalysts used by some bacteria to break into atoms the atmospheric molecular nitrogen, allowing biological nitrogen fixation. This, in turn allowed biologically driven nitrogen-fertilization of the oceans, and thus the development of more complex organisms. At least 50 molybdenum-containing enzymes are now known in bacteria and animals, though only the bacterial and cyanobacterial enzymes are involved in nitrogen fixation. Due to the diverse functions of the remainder of the enzymes, molybdenum is a required element for life in higher organisms (eukaryotes), though not in all bacteria. Characteristics In its pure form, molybdenum is silvery white metal with a Mohs hardness of 5.5. It has a melting point of 2,623 °C (4,753 °F); of the naturally occurring elements, only tantalum, osmium, rhenium, tungsten and carbon have higher melting points. Molybdenum burns only at temperatures above 600 °C (1,112 °F). It has one of the lowest coefficient of thermal expansion among commercially used metals. Tensile strength of molybdenum wires increases about 3 times from about 10 to 30 GPa when their diameter decreases from ~50–100 nm to 10 nm. Molybdenum is a transition metal with an electro negativity of 1.8 on the Pauling scale and an atomic mass of 95.94 g/mol. It does not react with oxygen or water at room temperature. At elevated temperatures, molybdenum trioxide is formed. Molybdenum has several oxidation states, the most stable being +4 and +6. The chemistry and the compounds show more similarity to those of tungsten than that of chromium. An example is the instability of molybdenum(III) and tungsten(III) compounds compared to the stability of the chromium(III) compounds. The highest oxidation state is common in the molybdenum(VI) oxide MoO3 while the normal sulphur compound is molybdenum disulfide MoS2. Molybdenum(VI) oxide is soluble in strong alkaline water, forming molybdates (MoO42−). Molybdates are weaker oxidants than chromates, but they show a similar tendency to form complex oxyanions by condensation at lower pH values, such as Mo7O246− and Mo8O264−. Polymolybdates can incorporate other ions into their structure, forming polyoxometalates. The dark-blue phosphorus-containing heteropolymolybdate PMo12O403− is used for the spectroscopic detection of phosphorus. The broad range of oxidation states of molybdenum is reflected in various molybdenum chlorides. Value The base value of each unit of ranges between 1 and 15Ð per unit, with up to 5 units being found at any one time. Presence on Mars: Common Category:Chemical elements